THIS INVENTION concerns turbochargers and more particularly a test rig for measuring out of balance conditions of a turbocharger core assembly.
The core assembly of a turbocharger comprises three main parts, namely a turbine which in service is driven by the exhaust gas from a pump, usually an internal combustion engine, a turbine wheel drivingly connected to the turbine whose function is to force more air into the pump's air intake or air supply, and a centre hub rotating assembly which contains bearings and a shaft directly connecting the turbine and turbine wheel. This core assembly can rotate in service at speeds typically in the region of 100,000 to 200,000 revolutions per minute and so dynamic balancing of the assembly about its rotational axis is critical. Individual imbalance of the turbine, the turbine wheel and the connecting shaft can cause an accumulation of balance error which at high operating speed can result in a noisy turbocharger and, in a severe case can cause premature bearing failure.
The imbalance can be corrected by running the core assembly at high speed on a flexible suspension of a test rig, measuring the vibration response and consequently removing metal from the rotational assembly to achieve an acceptable balance.
Such a test rig provides a slave turbine housing adaptor with quick release clamps for attachment and removal of the turbine core assembly, and the adaptor is in turn attached to a flexibly mounted air nozzle assembly which directs air into the housing to rotate the turbine and shaft. An accelerometer is attached to the adaptor to measure vibration of the complete assembly. Usually, the admission of air to the turbine is controlled allowing the assembly to be accelerated slowly across the appropriate speed range. Thus, the assembly may be balanced dynamically at different speeds.
Various methods and devices have been used for mounting and driving the turbocharger core assembly in a high-speed core balancing rig.
Such devices include, for example, a rubber or other resilient mounting block which isolates the assembly from a machine frame. A major disadvantage with resilient mounting of this kind is that the mounting block has degrees of freedom in all directions and does not properly manage the resilient support for the assembly.
Another technique is to mount the assembly in a rig comprising a pair of spaced leaf springs mounted on a rigid base so that the whole assembly is capable of oscillatory vibration in a single plane transversely across the springs. However, the air inlet is usually disposed at one side of the machine with the nozzle tube assembly passing between the leaf springs to transmit the air to the core assembly mounted at an opposite side. A disadvantage with this arrangement is that the nozzle tube assembly can introduce undesirable resonance resulting in an inaccurate measurement of the true imbalance vibration of the core assembly.
It is an object of the present invention to provide an apparatus for measuring rotational imbalance of a turbocharger core assembly wherein the aforementioned disadvantages are substantially avoided.
According to the present invention, there is provided an apparatus for measuring rotational imbalance of a turbocharger core assembly, comprising a base frame, support means for mounting a rotational core assembly thereon, air supply means for rotationally driving a turbine of the core assembly, and a device to measure transverse oscillatory vibration of the core assembly during rotation; characterized in that the support means comprises a rigid annular housing mounted between spaced resilient means attached to the base frame such as to afford a degree of movement of the rigid annular housing in a plane transverse to the rotational axis of the core assembly; and in that the rigid annular housing includes a volute through which air is fed to drive the core assembly.
Preferably, the spaced resilient means comprises a pair of leaf springs attached respectively at two opposed locations to the base frame such that they are located one on each side of the rotational axis of the core assembly.
The support means may be attached to and between the leaf springs.
The rigid annular housing of the support means may comprise two co-axial parts, the first part being attached by the resilient means to the base frame and having a driving air inlet passage communicating with an internal annular chamber; the second part being an adaptor removably attached to the first part and containing said volute which when the two parts are attached together extends into the annular chamber of the first part to receive driving air from said air inlet.
The air inlet in the first part may be bi-directional thus to enable air to be directed around said volute in which ever rotational direction is determined by the configuration of the volute, thus to enable selective directional rotation of a core assembly.
The volute may be configured to occlude one passage of the bi-directional air inlet of the first part while revealing the other for receiving driving air therefrom.
The volute may be configured to deliver driving air preferentially to a reduced diameter portion of the turbine of a core assembly.
The device to measure the transverse vibration of the core assembly may be an accelerometer mounted on the first part of the support means.
The base frame may comprise an annular air exhaust housing co-axially and sealingly engaged with the support means while allowing said degree of relative movement thereof.
Clamping means may be provided on the support means for ready attachment and removal of the turbocharger core assembly.